The geochemical variability of mantle-derived melts in both trace element and isotope ratios is such as to require a chemically heterogeneous source. This raises the need to understand the dynamics of melt segregation from a heterogeneous source and the associated effects of chemical fractionation and transport. A series of idealized numerical model problems, based on a two-phase flow representation of a partially molten source region, are used to address how the physical and geochemical properties of local heterogeneities contribute to the overall segregating melts. We find that when the heterogeneities represent material that melts more easily than the background depleted mantle they will give rise to discrete traveling pulses of high melt fraction that can be understood in terms of two-dimensional solitary waves, which we separately show are solutions to the governing equations used. The generation of solitary wave-like pulses by heterogeneous melting depends on the scale of the melting anomaly compared to the natural compaction length of the partially molten system, being most effective when the region of excess melting is large. In terms of the geochemical transport by solitary waves we find that they leave behind any distinctive trace element concentration associated with the region of more rapid melting that produces the wave and therefore that the physical and chemical effects decouple. The chemical transport of incompatible trace elements by solitary waves is still anomalous in the sense that while the trace element concentration becomes no different than that of the lower melt fraction background there is locally much more of the enriched fluid (melt) moving with a large phase velocity.

The general conclusion is that when a heterogeneous source partially melts it can give rise to fast moving melt fraction pulses or solitary waves, but these do not carry with them any chemical distinctiveness of the local heterogeneities that produce them. This suggests that the geochemical properties of mantle-derived melts should be interpreted in terms of averages over length scales comparable to the region of partial melting, and not particularly biased by local anomalous components even when these are preferentially melted during upwelling. ¿ American Geophysical Union 1989